Method of making a breathable, meltblown nonwoven

Details Method of making a breathable, meltblown nonwoven Method of making a breathable, meltblown nonwoven

1999-05-03

2001-01-23

Sells, James (Department: 1734)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S290000

Reexamination Certificate

active

06176952

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of making a breathable nonwoven fabric having enhanced moisture barrier properties. In particular, the invention pertains to a method of making a meltblown fibrous layer having an improved hydrohead performance (e.g. greater than 40 millibars (16 inches of H
2
O) and adjacent to at least one spunbond fibrous layer, wherein the method comprises secondary processing of the meltblown layer prior to bonding to spunbond layers. The resultant spunbond/meltblown (SM) nowoven fabric is breathable and characterized as having a cloth-like feel and softness and enhanced hydrohead performance rendering it suitable for use in, for example, the personal hygiene and medical markets for items such as infection control garments and coverings, incontinence pads and diapers, especially as a diaper backsheet or containment flap.
BACKGROUND OF THE INVENTION
Nonwoven fabrics used in disposal garments, diapers, incontinence pads and other personal hygiene items are required to possess a number of important end-use attributes. Key performance attributes include breathability, cloth-like feel and softness, drapeability and conformability as well as act as a barrier against the penetration of liquids. Clothlike feel and softness and conformability relate to wearer comfort and both attributes tend to correlate to the suppleness of the nonwoven fabric. However, breathability and barrier properties are inversely related since breathability relates to the comfort of the wearer by facilitating respiration. That is, good breathability refers to the passage o moisture vapor. Alternately, good barrier properties relate to the impermeability of liquids and bodily fluids such as blood in the case of surgical gowns and urine in the case of disposable diapers.
Known nonwoven fabrics and laminate structures represent a substantial performance compromise between breathability and barrier properties. That is, the art is replete with nonwoven fabrics that possess good breathability but low barrier performance and vice-versa. The art is also replete with various fiber making methods including meltblowing and spunbonding techniques as well as SMS structures. See, for example, U.S. Pat. No. 3,338,992 to Kinney; U.S. Pat. No. 3,502,538 to Levy; U.S. Pat. No. 3,502,763 to Hartman; U.S. Pat. No. 3,849,241 to Buntin; U.S. Pat. No. 4,041,203 to Brock et al.; U.S. Pat. No. 4,340,563 to Appel et al.; U.S. Pat. No. 4,374,888 to Bomslaeger; and U.S. Pat. No. 5,169,706 to Collier et al., the disclosures of all of which are incorporated herein by reference.
WO 97/34037, the disclosure of which is incorporated herein by reference, describes a laminate having at least one layer of meltblown elastic fibers bonded on either side with a layer of soft nonelastic fibers of greater than 7 microns in average diameter. All of the inventive examples in WO 97/34037 which consist of elastic meltblown layers exhibit a hydrohead performance less than or equal to 14.3 mbars. The exemplified control SMS structure in WO 97/34037 which consist of side-by-side polypropylene polyethylene spunbond layers and a nonelastic polypropylene layer exhibit a hydrohead performance of 21.3 mbars.
U.S. Pat. No. 5,607,798, the disclosure of which is incorporated herein by reference, describes a laminate which can be in the form of a SMS structure and comprises a polymer blend of a high crystalline polypropylene and a random block copolymer of polypropylene and polyethylene. The object of the invention described in U.S. Pat. No. 5,607,798 is said to be to provide a nonwoven fabric with improved strength properties. However, U.S. Pat. No. 5,607,798 provides no information respecting breathability and barrier performance of the described laminate. Significantly, U.S. Pat. No. 5,607,798 does not teach the specific or separate densification or recrystallization of meltblown layers.
WO 96/17119, the disclosure of which is incorporated herein by reference, spunbond and meltblown fibers made from metallocene catalyzed polyethylene wherein the polyethylene has a density greater than 0.940 grams/cm
3
. WO 96/17119 provides no hydrohead performance information for meltblown layers or SMS structures, does not describe specific or separate densification and/or recrystallization of the meltblown layers and only exemplifies meltblown layers having a basis weight of 68 grams/m
2
.
WO 97/29909, the disclosure of which is incorporated herein by reference, describes a clothlike microporous laminate made by incrementally stretching a lamination of a microporous film and nonwoven fibrous web. The laminate allegedly has air and moisture vapor permeabilities and acts as a barrier to the passage of liquids.
WO 97/30843 describes a fully elastic, breathable, barrier fabric comprising a nonwoven web layer of fibers of less than 40 microns in average diameter, wherein the web has a hydrohead performance of at least 10 millibars, a Frazier Permeability of at least 100 cfm, a basis weight of less than 68 g/m
2
and which is made from an elastic polymer e.g. ENGAGE™ elastomer supplied by Dupont Dow Elastomers. However, all of the inventive examples in WO 97/30843 show a hydrohead performance of less than or equal to 14 millibars. Further, the exemplified control SMS sample in WO 97/30843, which consists of all nonelastic layers, shows a high hydrohead performance and excessively low permeability. This performance is consistent with the expectations of a person skilled in the art. That is, nonelastic materials are ordinarily characterized as having higher crystallinities and high crystallinity is expected to provide good barrier properties e.g. high hydrohead performance but low permeability e.g. low moisture vapor transmission rates (MVTR).
WO 97/30202, the disclosure of which is also incorporated herein by reference, also describes an elastic meltblown layer. However, the hydrohead performance of the inventive examples 1 and 2 in WO 97/30202 are disclosed to be 5.2 and 7.2 millibars, respectively. Further, WO 97/30202 describes a comparative example 4 as a polypropylene/polypropylene/polypropylene SMS structure having a hydrohead performance of 33.6 millibars. However, the hydrohead performance of the meltblown layer is not disclosed nor is the exact basis weights for the individual layers. Conversely, the basis weight ratio between the spunbond and meltblown layers of comparative example 4 in WO 97/30202 is disclosed to be between about 1:1 and 1:4, i.e. the spunbond layers constitute about 20-50 percent by weight of the SMS structure.
Because there is no description in the art of a thermoplastic meltblown layer having good breathability and good barrier properties, there is a present need for such. In particular, there is a need for a thermoplastic meltblown layer characterized as having a basis weight less than or equal to 67 g/m
2
, a MVTR greater than or equal to 1,500 g/m
2
/day, and substantially improved hydrohead performance. There also is a need for a spunbond/meltblown (SM) structure characterized as having a cloth-like feel and softness, a basis weight in the range of from about 12 to about 105 g/m
2
, a MVTR greater than or equal to 1,500 g/m
2
/day, and a hydrohead performance greater than or equal to 45 millibars. There is also a need for a method for making the above described novel meltblown layer. There is a further need to provide a high barrier meltblown layer with good elasticity. These and other objects are met by the invention herein described.
SUMMARY OF THE INVENTION
We have discovered that by separately secondarily processing a thermoplastic meltblown fibrous web, the barrier properties of the web can be greatly enhanced while maintaining the high permeability of the web. Although we do not want to be held to any particular theory that might explain the invention, separate secondary processing of a thermoplastic meltblown web, for example, by thermally bonding the fibrous web between two smooth rolls at an elevated temperature and pressure and an effective residence time is believed to effectuate densif

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